When the Langley Variable Density Tunnel (VDT) was put into operation in 1922, its unique capability to test in pressurized conditions up to 20 atmospheres provided unprecedented aerodynamic data and made Langley a world-class aerodynamic research laboratory. This breakthrough facility could more accurately simulate flight conditions during tests of small-scale models than any other tunnel in the world. Langley quickly applied the tunnel to studies of the aerodynamic behavior of a variety of shapes including dirigibles, airplanes, and airfoils. The airfoil research, in particular, proved to be an extremely important contribution to the state of the art (see more on airfoils).

Unfortunately, while the VDT provided data in good agreement with flight test results for certain airfoil characteristics such as maximum lift, the levels of drag predicted by the tunnel testing were not in agreement with flight. Langley researcher Eastman Jacobs of the VDT staff had recognized that airstream turbulence in the VDT would be an issue, even during its early design stage. Artificial turbulence created by drive systems and flow quality shortcomings in wind tunnels can cause erroneous measurements in aerodynamic drag and other characteristics. Turbulence can affect the properties of the aerodynamic boundary layer (the thin layer of air adjacent to aircraft surfaces) which in part determines whether the airflow over components such as airfoils is laminar or turbulent. Jacobs’ concern was stimulated by two factors. First, he recognized that the geometry of the VDT was not conducive to low turbulence—in fact, the VDT had the highest levels of turbulence of any Langley tunnel at the time. Lowering the airstream turbulence requires a tunnel with a large contraction ratio (the ratio of settling-chamber to test-section cross-sectional areas) and flow-smoothing screens, and the VDT did not use either concept. The second factor of interest to Jacobs was that his emerging excitement for research on new low-drag airfoil shapes known as laminar flow airfoils required low turbulence levels for meaningful wind-tunnel studies. Aerodynamic drag of laminar flow airfoils is much less than that of conventional turbulent airfoils. Jacobs began an aggressive campaign to acquire a new wind tunnel capable of operation with extremely low levels of turbulence in its test section airstream.

Jacobs’ request for the construction of a new tunnel with "vanishingly low" turbulence levels was not accepted by Langley’s management, based in part on technical doubts over the necessity of such a facility by noted technical experts at Langley including Theodore Theodorsen. In addition, management believed that Congressional approval for the envisioned tunnel would be difficult to obtain based on the vague technical factors involved. However, aircraft icing problems were a high-priority issue in the NACA mission at the time, and Jacobs and Ira Abbott jointly designed an “icing tunnel” which had the same dimensions and layout as a desirable low-turbulence tunnel. Using this ploy, a full-scale model tunnel of a low-turbulence tunnel design was built in building 583 as the NACA Ice Tunnel with the stated purpose of investigating ice formation on aircraft components. In reality, it served the purpose of evaluating the revolutionary contraction and screening concepts required for low turbulence. The single-return, closed-throat tunnel was built of wood with a sheet steel lining. The Ice Tunnel became operational in June 1938 as an atmospheric (unpressurized) tunnel and served its alleged purpose with a few rapid investigations of icing, then its refrigeration equipment was removed and the tunnel was converted with arrays of honeycomb and screens to reduce the turbulence levels to extremely low levels. The tunnel was subsequently renamed the Langley Two-Dimensional Low Turbulence Tunnel (LTT). The highly successful operation of the model tunnel led to preparations to build a pressurized low turbulence tunnel to be known as the Langley Two-Dimensional Low Turbulence Pressure Tunnel (LTPT).

Authorization for the LTPT was granted in 1938 and the facility was constructed in Building 582A, becoming operational in May 1941. The tunnel had a test section 7.5' high by 3 ' wide and 7.5' in length. Its heavy steel plate structure could be pressurized to 10 atmospheres. The LTPT had a contraction ratio of 17.6 to 1 and 11 screen elements designed to smooth the airflow by breaking up eddies provided unprecedented low levels of turbulence. A 13' fan with 20 blades driven by a 2,000 horsepower motor provided test speeds up to about 380 mph (Mach 0.5). The flow circuit of the tunnel was built with curved sections rather than conventional 90° turns to minimize structural stress concentrations at the high air pressures used. Large splitter vanes were also used for structural reasons rather than flow control. Another unique feature of the tunnel was a metal sun shade over the top of the shell to reduce differences in temperatures between the upper and lower parts of the airstream due to solar heating. Differential heating of the airstream had been found to cause turbulence in preliminary testing of a smoke-flow tunnel. An airlock was provided to allow access to the test chamber and test section at pressures greater than atmospheric.

Modified blade removed for salvage

The political and economic atmosphere during construction of the tunnel is apparent in the choice of some of the construction materials. Several parts from P-38 aircraft were utilized as components in the facility. One particular example is the use of the aircraft propellers as fan blades. Originally, the NACA attempted to power the LTPT with unmodified P-38 propeller blades. The original blades could not absorb the power of the drive system and added too much turbulence to the airflow. This was attributed to the rounded tips of the P-38. The blades were modified by cutting the tips off. The solution was successful, and the modified blades continued to serve the tunnel until it was closed and the blades were removed for salvage as historic artifacts.

At the time of its introduction, the LTPT had the lowest turbulence levels of any wind tunnel in the world. Initially test programs in the new facility focused on further development of low-drag airfoils (NACA 6-series airfoils) that were incorporated into many U.S. military aircraft during World War II (the first laminar-flow airfoil that was used by the P-51 Mustang had been based on earlier data from the LTT). Coupled with extensive results from other facilities including the VDT and LTT, data from the LTPT were cataloged by Abbott and Von Doenhoff in a publication which became the “Bible” of airfoil information for many generations in the aeronautics community.

The LTPT was briefly converted for use with Freon in 1948 and was later modified with slotted walls to permit transonic testing in 1953, but neither of the modifications proved to be successful. After 1955 the LTPT served as a pressure vessel for the Langley 26" Transonic Blowdown Tunnel (TBT) located in nearby Building 583 (the previous site of the LTT). The TBT had been put into operation in 1950. The LTPT continued to be used as a pressure reservoir until the mid 1960s.

In the 1970s Richard Whitcomb's highly successful research on transonic supercritical airfoils and wings had stimulated the development of airfoils for low- and medium speed vehicles such as business jets and personal-owner general aviation aircraft. As part of a major Langley program on these topics, the LTPT was reactivated in the early 1970s when interest in the new airfoils peaked.

The LTPT underwent a major overhaul between December 1979 and March 1982. The two main objectives of the rehabilitation work were to restore and improve the flow quality required for laminar-flow research and to provide a two-dimensional model-support and force-balance for testing airfoils. During the overhaul activities the original anti-turbulence screens and a cooling coil used to remove the heat put into the airstream during operations were replaced. In addition to the model support and balance modifications, a sidewall boundary-layer control system was added as well as a remote-controlled survey apparatus and a new data acquisition system.

Turbulence measurements made in the test section after the modifications were completed were about the same as measurements made after initial operations began in 1941.

Research applications in the LTPT included extensive contributions of new airfoil designs, more efficient high-lift flap systems, the first experiments with active laminar-flow-control concepts, and basic studies of flow phenomena at high Reynolds numbers. Aerodynamic studies of specific aerospace vehicles have contributed to the development of the X-1, F-100, F-111, F-14, F-15 and C-5A aircraft; and the Saturn, Apollo and Space Shuttle spacecraft. Late research had focused on problems of efficiently integrating engines into airframes for better performance of commercial and military aircraft.

In 2006, the drive motor of the LTPT burned, resulting in termination of operations. A decision deactivate the tunnel was made as no funds were available to refurbish the tunnel. Plans for demolition of the LTPT have begun. Some parts have already be removed for preservation. The remainder of the building will be removed in 2013. See Salvage List.

Demolition of the interior of the building began in the fall of 2013. The following photos were captured by Kevin Schrock with All Phase Services, one of the demolition contractors. These show some of the items that he found more interesting.

Additional photos taken in Spring and early Summer 2014 show the exterior demolition in progress: